At the present rainflow-counting and its variants are the most widely used methods in the analysis of fatigue data of mechanical components. The rainflow-counting method allows assessing the fatigue life of a structure subjected to a complex dynamic mechanical loading, the assessing being based on counting numbers of load half-cycles. The rainflow-counting method was originally developed in: Matsuiski, M. and Endo, T. Fatigue of metals subjected to varying stress, Japan Soc. Mech. Engineering (1969). A variant of the rainflow-counting method, as described in: Downing, S. D., Socie, D. F. (1982) Simple rainflow counting algorithms. International Journal of Fatigue, Volume 4, Issue 1, January, 31-40, is used for the fatigue analysis of wind turbine components.
In order to apply the rainflow-counting method to the fatigue analysis of wind turbine components, measurements of component loads with a given sampling frequency are performed during the life-time of the wind turbine. Thereby, measurements of mechanical loads obtained in a response time, i.e., a predefined time period within the life-time of the turbine, for instance, a time period of 10 minutes are collected and used to obtain the time dependence of the mechanical load during the response time, the time dependence being represented by the corresponding discrete load curve, i.e. sample curve. The whole sample curve is used to determine the numbers of load half-cycles belonging to individual bins. This means that all measurements of mechanical loads performed during the response period are used. The numbers of load half-cycles belonging to individual bins occurring during the response time are then used to update count values of numbers of half-cycles belonging to individual bins occurring from a starting time of the counting. The starting time may be the time when the wind turbine has been brought into operation. Hence, the rainflow-counting method exhibits a severe drawback, which is inherent to the rainflow-counting method because when using this method, it is not possible to update the count values of numbers of half-cycles with each new sample, i.e., with each new load measurement.
It may be an object of the present invention to provide an efficient and reliable mechanical load determination for a component, in particular for a component of a machine, which component is subjected to a dynamic mechanical loading.